Fix N+1 queries loading milestones when exporting CSVs

During exporting of CSVs, we weren't preloading milestones, so every
issue had to load its related milestones.

app/services/issues/export_csv_service.rb

@@ -34,7 +34,7 @@ module Issues
     private
 
     def associations_to_preload
-      %i(author assignees timelogs)
+      %i(author assignees timelogs milestone)
     end
 
     def header_to_value_hash

changelogs/unreleased/sh-export-csv-service-nplus-one.yml

+---
+title: Fix N+1 queries loading milestones when exporting CSVs
+merge_request: 49429
+author:
+type: performance